It's really weird. And I have logged a fault with NBNZ about it as it seems to be firewall weirdness on their side as I have duped this with 5 different internet connections (TCL Cable, Telecom XT, Vodafone Mobile Data, Work proxy server in the US, Work proxy server in NZ going via Orcon).

I think they have an IDS or some sort of door knocking style login process that requires you to first connect to the cleartext port on http then redirect to the https port afterwards. And not go direct to the HTTPS port.

Waiting 15-30 mins seems to kick it back into life if I have first accidently clicked on the https url before going to the http first.